I think it's time to put things straight about one of the reasons(cultural in this case) why DE is viewed as "evil" with a simple comparison between our western PoV regarding good and evil and "political values":

See the topmost corner? It represents
authoritarianism, or in our case, something constantly associated with evil. When we speak about it we are speaking about a system of social control based on strict obedience to a form of authority with strong control of people's life by it, it also relates with police states, dictatorships, absolutism, oppression, obedience, loyalty till death. We westerns(the majority at least) certainly scorn the values associated with ideologies on this spectrum. From our cultural standpoint it can be quickly associated with the concept of evil. A coercive and oppresive organization subduing people's will and smashing their liberties won't be seen as a good thing.
Now on the other diametrically opposite of the bottom of that compass lies
libertarianism, where it is believed government shouldn't interfere with people's lives, and is supported the supremacy of individual rights and personal freedoms over any kind of authority, associated with freedom of speech, freedom of press, freedom of choice among others many of us are used with and even more(pro-legalization of marijuana among other liberal stances over polemic questions). This position may be filled with the so-called archetypes of the "Freedom Fighters", and promote their ideals of freeing a land from despotism or struggling to achieve even more civil liberties in an already mostly liberal environment. This people, in a quick glance, can easily be associated with the concept of good in our eyes(except if religious values go against their libertarian principles).
So here it sums up:
Dark Empire is unarguably a very
authoritarian organization, including on its own ideology, and from the OOC point of view of the majority of the Western World(that is sometimes very difficult to remove completely because of its cultural origins), being authoritarian equates with being evil, therefore the Dark Empire is
"evil".
And to make things even more accentuated, the commonly remembered connotations of the word "Imperialism" aren't very positive either, as they are related with ruthless conquer of foreign lands. The "Dark" is not the main source of this view, in my opinion.
"Ambition to achieve what is possible" and "World Domination" can be synonymous, if an organization becomes sufficiently powerful for it. And values aren't so solid at all on groups seeking political power. Ambiguous statements might make people assume the worst.
An I will be honest, when I try to think how would the Dark Empire work once it became a nation, first things I remember are 1984 among other dystopian novels...
Words have power, Here is an Example: Norsefire is "good" on all its propaganda efforts, but in fact all its working lies on tyranny and oppression, and once a masked man comes to announce and broadcast the truth, things start to slowly change...
But
I see the Dark Empire more akin on its principles to other fictional group: the PlanetSide's
Terran Republic, and it
also is associated with evil while the "freedom fighters" of the "New Conglomerate" there are considered by its players as the "good guys".
Now think about it: a dystopian empire isn't something so bad from OOC perspective, specially due to the fact things are currently very bland and uninteresting around Yliakum. We need some tough to overthrow oppressors so we can have some focus instead of wandering around chit-chatting. If the Dark Empire fails on presenting an interesting perspective on that, I would be tempted to push a straight-forward(no disguises of its nature) "Orwellian" guild to make something akin to a very small version of IngSoc, but more medieval.
Finally, there are some "empires"(read: countries that seek domination) that use the values of freedom, democracy and liberty as pretexts to invade and "free" other countries ruled by dictators, but this is another history...